As a light source for optical fiber communications and optical interconnection, a laser device has been proposed which has a structure where an optical semiconductor device, which is made of a compound semiconductor, and a silicon (Si) optical waveguide are hybrid integrated. Such a laser device has features to be operated with less energy consumption and to be formed in smaller size. As a light modulation device using such a laser device, there is a light modulation device having a structure of a laser device 900 and a ring assist optical modulator 960 as illustrated in FIG. 1.
As illustrated in FIG. 2, the laser device 900 includes an optical semiconductor device 910, which is called a Semiconductor Optical Amplifier (SOA), and a wavelength-selective reflection device 950 in which optical waveguides are formed. In the optical semiconductor device 910, there are formed a gain waveguide 911. Further, an antireflection film 912 is formed on one surface which is one of end surfaces of the gain waveguide 911. From the one surface of the gain waveguide 911, light is emitted. Further, a high reflection film 913 is formed on the other surface which is the other of the end surfaces of the gain waveguide 911.
The wavelength-selective reflection device 950 includes a first optical waveguide 951, a ring resonator 952, a second optical waveguide 953, and a wavelength-selective reflection mirror 954 which is formed in the second optical waveguide 953. The optical semiconductor device 910 and the wavelength-selective reflection device 950 are installed in a manner such that an emission end 911a of the gain waveguide 911, which corresponds to the one surface of the optical semiconductor device 910, and an incident end 951a, from which light is incident into the first optical waveguide 951 of the wavelength-selective reflection device 95, face each other.
Further, as illustrated in FIG. 1, the ring assist optical modulator 960 includes a first modulator optical waveguide 961 and a second modulator optical waveguide 962. The first modulator optical waveguide 961 and the second modulator optical waveguide 962 are connected to each other on the light incident side by an optical waveguide 965 and on the light emission side by an optical waveguide 966. Further, near a side part of the first modulator optical waveguide 961, plural ring resonators 963 are provided. Further, near a side part of the second modulator optical waveguide 962, plural ring resonators 964 are provided.
In the laser device 900, a resonator is formed by the high reflection film 913, which is formed on the other surface of the optical semiconductor device 910, the ring resonator 952, which is formed in the wavelength-selective reflection device 950, and the wavelength-selective reflection mirror 954. Accordingly, in the laser device 900, by flowing a current, etc., in the optical semiconductor device 910, light is emitted. Then, the emitted light, which is emitted in the optical semiconductor device 910, is laser-oscillated by the resonator, etc., so that laser light is emitted from an output end 953a (FIG. 2) of the second optical waveguide 953.
The laser light, which is emitted from the wavelength-selective reflection device 950, is incident in the ring assist optical modulator 960, and travels through the optical waveguide 965 which is formed on the incidence side. The laser light, then, is divided into laser light to travel through the first modulator optical waveguide 961 and laser light to travel through the second modulator optical waveguide 962. In the ring assist optical modulator 960, it is possible to change the phase of the traveling light by, for example, applying a voltage to the first modulator optical waveguide 961 or the second modulator optical waveguide 962. After that, the laser light traveling through the first modulator optical waveguide 961 and the laser light traveling through the second modulator optical waveguide 962 are coupled, and the coupled laser light is output from an output end 966a of the optical waveguide 966 which is formed on the emission side. Here, the laser lights, which propagate through the first modulator optical waveguide 961 and the second modulator optical waveguide 962 are coupled at the optical waveguide 966. The laser light intensity emitted from the output end 966a is depend on the phase difference between the first modulator optical waveguide 961 and the second modulator optical waveguide 962. Therefore, the laser light emitted from the output end 966a modulated. Further, in the optical modulator having the structure of FIG. 1, the wavelength-selective reflection device 950 and the ring assist optical modulator 960 are formed on the same silicon substrate 970.
In such a ring assist optical modulator 960, an operation wavelength band where light modulation can be performed efficiently is narrow such as approximately 1 nm near the resonance wavelength. Therefore, it is desired that the oscillation wavelength of the laser device as a light source is the same as the resonance wavelength of the ring(s) of the ring assist optical modulator 960. As a technique to realizes such a relationship between the oscillation wavelength and the resonance wavelength, the laser device 900 as described above is disclosed which includes the optical semiconductor device 910 and the wavelength-selective reflection device 950 and the ring assist optical modulator 960 which are formed on the same silicon substrate 970. Further, the ring resonator 952, which is in the wavelength-selective reflection device 950, and the ring resonators 963 and 964 are formed in a manner such that the ring resonators 952, 963, and 964 have the same structure.
In such a laser device 900, the oscillation wavelength of the laser light can be determined by selecting one of the transmission center wavelengths, which are periodically spaced apart from each other, of the ring resonator 952 by using the wavelength-selective reflection mirror 954 in the wavelength-selective reflection device 950.
In the case of FIG. 1, as described above, the ring resonator 952 and the ring resonators 963 and 964 are formed so as to have the same cycle length. In such a structure, transparent wavelengths of the ring resonator 952 coincides with that of the ring resonators 963 and 964 are automatically unified. Therefore, it becomes possible to emit the laser light having the operating wavelength of the ring assist optical modulator 960 from the laser device 900 without any specific control. Further, in a case where the wavelength-selective reflection device 950 and the ring assist optical modulator 960 are formed on the same silicon substrate 970, even when a temperature of the silicon substrate 970 changes, the operations are obtained. Namely, even when a temperature change of the silicon substrate 970 occurs, the temperature is increased or decreased in the silicon substrate 970 as a whole. Therefore, the waveform shift occurs in the same manner in both the ring resonator 952 and the ring resonators 963 and 964. Therefore, it is possible to maintain a stable operation even when the temperature of the silicon substrate 970 changes.
Reference is made to Japanese Laid-open Patent Publication No. 2011-253930.